This invention relates to pressure transducers and more particularly to compensating arrangements of such transducers in order to substantially reduce nonlinearities.
The use of the piezoresistive effect in semiconductors has made possible the construction of a wide variety of transducers which exhibit enchanced outputs and operating frequencies over a wide range of temperature and pressure conditions.
Such devices employ the most advanced techniques for fabrication as developed and utilized in the field of solid state technology. These techniques enable size reduction, excellent repeatibility and reliability, high output, lower power dissipation, while providing an overall simplification of design and application.
In the semiconductor transducer the piezoresistive effect exhibited accounts for a change in the resistance of the strain gage in response to a change in pressure. In monolithic transducers the piezoresistive effect may be due to the longitudinal or transverse piezoresistive effect.
Apart from the advantages as described above, there is a problem associated with such transducers operating with either effect.
The combined effects of the nonlinearity of the pressure versus stress function of the mechanical element used as a force collector and the basic piezoresistive nonlinearity combine to produce a pressure sensor with a nonlinear behavior.
The longitudinal piezoresistive effect is widely used and most transducers in the prior art relied on this effect for operation. When employing the longitudinal effect, the transducers typically exhibit a positive nonlinearity over a pressure range. Essentially a transducer should operate to provide a linear output upon application of varying pressure. Over the full range of operation this should be a straight line between the end points as from zero applied pressure to full scale. Longitudinal devices usually exhibit a positive nonlinearity which provides a relatively straight line operation above the theoretical end points. The nonlinearity is due to many factors which prevent true end point operation as from zero to full scale.
For instance in a high pressure sensor shear stresses on the diaphragm support act to decrease stress levels at peripheral gage elements. In low pressure transducers membrane stiffening effects produce a positively nonlinear behavior. These effects combine with the inherently positive nonlinear response of a longitudinal piezoresistive element to produce a postively nonlinear pressure transducer.
Transducers employing transverse piezoresistive sensors however may be positively or negatively nonlinear. The basic transverse piezoresistive coefficient is negatively nonlinear for tensile stress and positively nonlinear for compressive stress. Thus, depending on the mechanical flexure nonlinearity and the placement and operating stress levels of the transverse elements a pressure sensor with a negatively nonlinear characteristic can be readily fabricated.
The pressure variations are in part due to the particular construction of the pressure responsive diaphragm as well as the construction of the individual sensors or gages. These variations of the output of the device with respect to pressure is not related to temperature and does not vary according to the same relationship as governing temperature changes. Hence the manufacturer desirably would like to compensate for these pressure variations and to eliminate the same. The elimination of such effects is extremely difficult. For example, there is variation in pressure transducers which manifests itself in a non-linear output voltage of one volt for an applied pressure of one psi. In this manner, the bridge should provide an output of one-half volt for an applied pressure of one-half psi. A typical piezoresistive bridge arrangement which is not compensated will not do this and will not provide a linear output pressure according to applied increments of reference pressure. This variation is not linear but is of the following relationship: EQU Y=AP+BP.sup.2 +CP.sup.3
where
Y=output voltage of the bridge. PA1 P=applied pressure. PA1 A is a constant unique to the bridge configuration. PA1 B is another constant particular to the bridge configuration. PA1 C is a third constant particular to the bridge configuration.
As indicated there is great difficulty in compensating for such variations in pressure which cause the above described nonlinearities. For example, see U.S. Pat. No. 4,192,005 entitled COMPENSATED PRESSURE TRANSDUCER EMPLOYING DIGITAL PROCESSING TECHNIQUES issued on Mar. 4, 1980 to Anthony D. Kurtz and assigned to the assignee herein.
This patent describes the nonlinearities and operates a transducer in conjunction with digital processing circuitry to provide a compensated output over a wide range of both pressure and temperature.
As described above, transducers exhibiting the longitudinal piezoresistive effect exhibit a positive nonlinearity, while those employing the transverse piezoresistive effect exhibit a negative nonlinearity. The negative nonlinearity appears below the theoretical response line between end points while, as indicated, the positive nonlinearity is above the response line.
It is an object of this invention to provide a compensated transducer structure capable of providing an improved linearity over a wide range of pressure variations. The transducer to be described is simple to construct and fabricate and provides compensation over a wide pressure range.